An FBI agent who participated in secret break-ins at the residences of friends and relatives of members of the Weather Underground testified in federal courts here yesterday that the searches were no help in either apprehending Weathermen fugitives or solving a series of bombings for which the radical group had claimed credit.

The witness, James Vermeersch, was a member of Squad 47, a team of as many as 50 FBI agents from the bureau's New York office. They were assembled in March 1970 to stop the bombings and to track down Weathermen sought on federal arrest warrants.

Vermeersch testified for the government at the trial of two former high-ranking FBI officials, W. Mark Felt and Edward S. Miller. Both are charged with conspiracy to violate the civil rights of the persons who were the targets of the break-ins, which were carried out in New York and New Jersey in 1972 and 1973. The charges of 10 years in jail, a $10,000 fine or both.

Vermeersch, a lawyer who is now legal adviser to the bureau's St. Louis office, said that while he was a member of Squad 47 he participated in 15 to 20 "black bag jobs" -- FBI language for surreptitious entries without a warrant.

The bombing and the pursuit of the Weathermen fugitives, he said, presented "a unique threat to the bureau. All we knew when the squad was formed was that the bombs were exploding and no one knew who was responsible." As a result, Vermeersch testified, Squad 47 focused on persons it believed were connected to fugitives but who were "in fact living as normal citizens above ground."

The government contends that the entries, approved by Felt and Miller, were illegal because they targeted persons who had not been charged with any crime. Defense lawyers told the jury they will prove that Felt and Miller authorized the break-ins for national security reasons to stop terrorists activities by the Weathermen, who they reasonably believed had significant connections with hostile countries. The government says that even in that case, Felt and Miller did not have the authority that was needed from either the president or the attorney general to authorize the entires.

Vermeersch, who participated in break-ins at four of the five residences listed in the indictment against Felt and Miller, said that the agents were searching not only for clues as to the whereabouts of the fugitives, but also for leads to the people behind the bombings. For example, Vermeersch testified, typewriting specimens found in the residences were examined to see if they matched typewriting on notes in which the Weathermen had claimed responsibility for the bombings.

However, he testified, none of the information collected ever led to an arrest. Its only value, he said, was to help the bureau "structuralize" the Weather Underground.

Vermeersch told the jury that agents usually obtained apartment keys from landlords who were paid about $50 for their cooperation. Otherwise, an agent who was trained to pick locks obtained entry, Vermeersch testified. Inside, the agents hurriedly searched "just about everywhere" and then photographed documents with a camera concealed in an attache case.